The inner surface of the light-source or lamp envelope should often be covered by a layer prepared from materials having a definite graininess. The layer prepared should be able to provide diffuse light, to modify the spectrum of the light emitted by the filament, or to transform the light of an ultraviolet spectrum emitted by a gas discharge lamp into light of the visible spectrum.
The most modern method of producing such layers is by electrostatic spraying. Apparatuses for electrostatic spraying are described for example in the U.S. Pat. Nos. 2,806,844; 2,881,131; 2,921,827; 3,017,852; 4,081,709; and 4,099,080.
Electrostatic spraying is in essence a method, whereby the inner surface of the light-transmissive envelope is covered by a dust formed from an inorganic material. The dust grains are transformed into a smoke form and the smoke particles are given an electrostatic charge in a desired way and blown into the glass envelope made electrically conductive by heating. The glass envelope is connected with one of the electrodes by the flame used for heating while the other electrode constitutes part of the means used for electrostatically charging the dust particles. When the smoke particles meet the surface of the glass vessel an electric circuit is closed between the electrodes.
The U.S. patents cited above relate to various units for the practical realization of the electrostatic spraying method in lamp producing plants; in this way the use of hydrofluoric acid for making the envelopes translucent or the use of fluid suspensions for this purpose can be avoided at the inner surface.
U.S. Pat. No. 3,961,600 describes an apparatus in which the dust is blown into the envelope in four positions, one after another. This apparatus can provide a layer of a desired thickness and high uniformity; however, productivity of production is limited by the number of positions.
U.S. Pat. No. 2,017,852 describes an apparatus for electrostatic spraying in which there are two times six stationary positions for spraying with the envelopes being coated alternately. This renders high productivity possible, however the positions are stationary and therefore there is a difference between the thickness values of different layers which is impossible to correct later.